In order to provide push-button dialing service in those telephone systems which are incapable of responding to multifrequency dialing tones, telephone circuitry is required to convert each push-button input to a conventional series of dial pulses which are recognizable by central offices. Such circuitry must convert each push-button input into a series of breaks, or interruptions, of the line current that are transmitted on the telephone line from the station apparatus to the central office, the number of such breaks being equal to the digit dialed. These make-break intervals must occur at standard repetition rates with standard make-break ratios so that the central office can properly detect the digit dialed.
When dial pulses are not generated by the electro-mechanical action of a rotary dial but, instead, are electrically generated in response to a push-button input, a pulsing element is required to interrupt the line current at the appropriate rate. One technique is to place the pulsing element across the tip and ring leads of the telephone line. During dialing, the pulsing element opens and closes the telephone line, while the other telephone circuitry is isolated from the telephone line. After dialing, the pulsing element is disconnected from the tip and ring leads, thereby preventing any deleterious effects on the voice signal. A disadvantage of isolating the telephone circuitry from the pulsing circuitry during the dialing interval is that power cannot be readily derived from the telephone line during each make interval.
An alternate technique for generating dial pulses is to place a pulsing element in series with the telephone line and the rest of the telephone circuitry. Dial pulses are generated by repetitively turning the pulsing element ON and OFF which creates breaks in the line current. During each make interval, the telephone line is connected to the telephone line and power can be derived therefrom. In U.S. Pat. No. 3,932,707, dial pulsing circuitry is disclosed in which a relay is employed to pulse the line current. A relay, however, requires more power than may be available, if the telephone is completely line powered.
A transistor, in series with the telephone line and the other telephone circuitry, is a preferable pulsing element for creating dial pulses by pulsing the conduction state of the transistor, since the circuitry can be powered totally by the telephone line. During nondialing intervals, this pulsing transistor must be biased to be in the conductive ON state. Enough base current must, therefore, be supplied to keep the pulsing transistor ON. The amount of base current necessary to saturate the transistor, however, is a function of the emitter-collector current through the transistor and the .beta. gain coefficient of the transistor. The emitter-collector current is essentially the line current, which can vary between two milliamps and in excess of one hundred milliamps, depending upon several factors, including, most significantly, the length of the telephone loop. In order for the pulsing circuitry to operate in all installations, enough base current must be supplied to saturate the pulsing transistor for the maximum possible line current. Typically, a transistor is biased through resistive circuitry connected to the base. Resistive circuitry small enough to provide enough base current to saturate the transistor for all possible variations of line current will, however, provide an alternate path for the voice signal resulting in attenuation of the signal, thereby affecting the quality of service provided.
In U.S. Pat. No. 4,008,379, a circuit is disclosed in which two transistors in a standard Darlington configuration, are connected in series with a telephone line for dial pulsing purposes. By using a Darlington pair, a larger biasing resistor can be used in the base circuit for biasing than would be required for use with a single pulsing transistor. However, such a circuit still might not provide a sufficiently high AC impedance across the telephone line to ensure negligible attenuation of voice signals. Furthermore, there is a significantly higher voltage drop across a Darlington pair than across a single transistor, which limits the voltage available to the rest of the telephone circuitry when there is a fixed total voltage requirement.
As an alternative to resistive biasing of a pulsing transistor, a current source can be employed to provide sufficient base current to the transistor to keep it saturated. A current source has essentially infinite AC impedance, and will not cause attenuation of the voice signal. Because of the wide variation in line current, as discussed hereinabove, a current source designed to supply sufficient current to saturate the pulsing transistor in the presence of the maximum possible line current will draw two much current in those network situations when the line curent is at its minimum possible value. As a result, insufficient current will be supplied to the rest of the telephone circuitry, thereby inhibiting its operation.